The invention relates generally to apparatus for the treatment of hypothermia, and more particularly to portable heated oxygen systems for administering to the profoundly hypothermic victim.
Hypothermia is one of the most frequently encountered and yet often overlooked aspects of emergency medicine. Accidental hypothermia is defined as "a condition in which the core body temperature is less than 35.degree. C. (95.degree. F.) as a result of accidental exposure", and is to be distinguished from intentional hypothermia deliberately induced as a medical procedure. However, hypothermia like other medical conditions is gradated by the degree of severity and the symtoms and urgency of treatment may differ radically at different levels. In mild hypothermia above 35.degree. C. (95.degree. F.), the accepted treatment is passive rewarming by natural or endogenous heat generation, which is simple, requires no equipment (other than a warm environment and/or blanket), and has no inherent morbidity. Moderate hypothermia occurs in the range of 32.degree. to 35.degree. C. (89.6.degree. to 95.degree. F.) and may produce loss of motor control, slurred speech and amnesia; marked hypothermia occurs at body core temperatures from 28.degree. to 32.degree. C. (82.4.degree. to 89.6.degree. F.) and is indicated by muscle rigidity, peripheral cyanosis and shock; and severe hypothermia occurs at temperatures from 25.degree. to 28.degree. C. (77.degree. to 82.4.degree. F.) at which the victim may have lost deep tendon reflexes and ventricular fibrillation and may appear dead with no palpable pulse or audible heartbeat. Body temperatures below 25.degree. C. (77.degree. F.) cause cardio-pulmonary arrest and death. These forms of hypothermia clearly present life-threatening medical problems complicating the treatment of the victim.
It seems surprising that the medical and paramedical fields have been slow in undertaking the intensive study of the nature and scope of cold stress, and its debilitating effect on the human body depending upon the relative severity resulting from exposure and traumatizing conditions coupled with the physical variables of individuals. Of course, the problems inherent in treating hypothermia victims are generally compounded in accidental hypothermia cases because most of these naturally occur to outdoor people, such as swimmers, campers and hikers, at remote locations in nature. In short, cold air and water exposure have been recognized for a long time as the primary potential for rapid heat loss, and various first-aid or emergency medical therapy for the profoundly hypothermic victim have been proposed.
Other than passive rewarming in the case of mild hypothermia, the traditional treatment of accidental hypothermia has been active external or peripheral warming. This involves the application of heat to the external body surface as by thermal blankets, heated bath emmersion, heat packs and the like, but most of these techniques are not available in the field at the rescue site. Furthermore, rapid external rewarming can produce various "rewarming shock" conditions that can be critical. The hypothermia victim is frequently dehydrated, the neuro-regulatory systems are functioning slowly, and the blood circulatory metabolism is not balanced. Thus, it should be recognized that "afterdrop" frequently results from external rewarming techniques in which the core temperature continues to drop (the cold at the periphery being driven inwardly) as the outer body is rewarmed. Obviously, cooling of the heart is critical since a reduced heart temperature of 28.degree. C. (82.degree. F.) can lead to total arrest and it is difficult to defibrillate a cold heart. In short, rapid peripheral or external rewarming of the accidental hypothermia victim may create additional hazards that become fatal and, at best, cannot be managed effectively in a non-hospital setting.
It is becoming established that the safest and most efficient technique for treating the severely hypothermic victim is by active core rewarming, i.e. the delivery of heat primarily to the body core or central circulation system (and also avoiding simultaneous rapid rewarming of the skin and extremities). The physiologic concept here is that if the heart, brain and lungs warm first, their control of perfusion, oxygenation and heat distribution will minimize core temperature afterdrop and the corporal metabolic hazards. A large number of hospital therapy techniques are available for rapid core rewarming, including heated peritoneal dialysis, heated gastric lavages, heated hemodialysis or intravenous fluids, extra corporeal circulation by femoral access (heart-lung machine) and inhalation rewarming. Only inhalation rewarming is a suitable technique for use by paramedics and other trained emergency rescue personnel at a rescue site or during transport to hospital or clinical facilities. Hayward et al U.S. Pat. No. 4,319,566 is relevant to inhalation rewarming and stresses the importance of delivering warm, water-saturated air or oxygen directly to the head, neck and thoracic core as the strategic body area to minimize "afterdrop" of core temperature without stimulating return of peripheral blood with high acidity and potassium concentration. However, while Hayward et al '566 approaches the problem of core rewarming, it teaches a steam heat generating apparatus for breathing patients only and one that is impractical at best in emergency rescue efforts in the field--the emphasis in this patent is on humidification as well as warming of air, and humidification itself creates problems systemically in a non-breathing patient as well as in the functioning of certain resuscitation equipment.
In addition to the publication literature cited in Hayward et al '566, some other reports of inhalation rewarming techniques include "Emergency Management of Accidental Hypothermia" by Rodney D. Edwards, M.D.; "Resuscitation From Hypothermia: A Literature Review", Report No. CG-D-26-79, Final Report--February 1979, by U.S. Department of Transportation (U.S. Coast Guard); "SASS Hypothermia-Cold Water Survival" (Critical Care Supplement), 1983, by David S. and Sara J. Smith. In addition to patent literature cited in Hayward et al '566 are U.S. Pat. Nos. 923,751; 946,307; 999,950 and 4,197,842.